1. Field of the Invention
The present invention relates to a system-for reproducing a digital signal which is recorded in a predetermined recording medium, and more particularly, to a digital playback signal detecting method and apparatus thereof which can facilitate data restoration at maximum by supplementing a function of a data detection circuit which compensates for damaged and distorted playback signals, and which can detect data stably even in a signal having a low signal-to-noise ratio.
2. Discussion of Related Art
In a general digital magnetic recording and reproducing system such as a D-VCR and DAT for recording and reproducing a digital signal, an input analog signal is recorded on a recording medium such as a magnetic tape with a digital signal, and then a playback digital signal, during reproduction, is restored into a digital signal which is the same as that during recording, to thereby convert the restored digital signal into an analog signal. In this case, when restoring data of the playback signal, the error rate is increased by intersymbol interference and signal-to-noise ratio degradation primarily due to the characteristics of a rotary transformer, which is located between a magnetic head and an amplifier, and a recording medium. Accordingly, the overall performance of the system is lowered. To prevent the above defect, phase correction and amplitude correction with respect to the playback signal have been conventionally performed to then detect the data.
FIG. 1 is a block diagram showing a conventional digital playback signal detection apparatus, which includes a playback amplifier 11 for amplifying digital data playback from a recording medium via a playback head (not shown) into a signal of a predetermined magnitude. A first equalizer 12 for correcting phase of the amplified playback signal is connected to the output of playback amplifier 11. A second equalizer 13 having a structure of a transversal filter is connected to the output of the first equalizer 12. In more detail, second equalizer 13 includes a 3-tap equalizer having two delays 14 and 15, in each of which the tap is formed at a predetermined time of interval T, and an adder 16 for adding the output signals of the respective taps. Variable resistors are connected between each of the outputs of the taps and adder 16, respectively.
In FIG. 1, the digital data which has been recorded on a predetermined recording medium such as a magnetic tape is reproduced via a playback head (not shows). The playback signal is input to playback amplifier 11 and amplified into a signal of a predetermined magnitude. The amplified playback signal is input to first equalizer 12 to correct phase distortion. The phase-corrected playback signal is input to a first delay 14 and delayed by a predetermined time interval T. The phase-corrected playback signal is also adjusted by gain K.sub.3 according to a resistance value of a variable resistor and the gain-adjusted signal is output for adder 16. The first delayed playback signal is input to second delay 15 and delayed by a predetermined time interval T. The first delayed playback signal is also adjusted by gain K.sub.2 according to a resistance value of a variable resistor and the gain-adjusted signal is output for adder 16. The first delayed playback signal is also adjusted by gain K.sub.1 according to a resistance value of a variable resistor and the gain-adjusted signal is output to adder 16. That is, the outputs of the respective taps do not suffer from the intersymbol interference by adjusting the resistance values of the variable resistors to change the respective gains. Adder 16 adds the outputs of the respective taps for which the gains are adjusted and, thus, improves asymmetry of a playback isolating waveform, peak-shift and amplitude degradation. Thereafter, the added signal is output to a data detector, to detect the originally recorded data. However, the conventional detection apparatus can perform correction of only a slight asymmetry and peak-shift as well as the amplitude correction, or can perform correction of only the slight peak-shift and amplitude as well as the asymmetry correction. However, the conventional equalizer cannot perform an ideal equalization at all the portions. Since the number of the taps in the transversal filter must be increased in an infinite number in order to perform an ideal equalization, it becomes difficult to adjust the gain of every tap. Also, in case of introduction of the automatic equalization algorithm, hardware implementation is burdensome.
On the other hand, a conventional differentiation-based 3-tap cosine equalizer is disclosed in a Japanese TV Journal volume 40, No. 6, 1986, "Differentiation-based Waveform EQ In High Density Digital Magnetic Recording." However, this circuit does not include an element for performing an integration function and a differentiation function needed to reduce the error rate. As a practical matter, if the tape is degraded, the state of the system becomes unstable, and the signal-to-noise ratio of the playback becomes extremely low, thus, exact data cannot be detected.